Captain Beeky <[EMAIL PROTECTED]> wrote: >On 24 Feb 2008, at 10:47, Michael Askin wrote: > >> Running a propeller when free running is much lighter work than when >> moored or towing > >I have a problem with this concept. How does the engine know what is >going on "out back" ? > >It turns the prop and pushes water backward or forward. I can see that >it would work harder if stirring treacle rather than water, and less >hard if running in air. > >I suppose when underway in forward gear the water being pushed >backwards by the prop already has some relative backward motion >relative to the prop . . . .
The amount of energy required to move a boat, and thus how hard the engine must work, depends on how fast the propeller is trying to move the water it is chewing through compared to how fast the boat is moving through the water. If the boat has a fairly slippery shape, and is moving well below its hull speed, and not accelerating, in slack water and a commodious channel, the propeller will be trying to move water through the prop very little faster than the speed of the boat. The engine won't need to work very hard. In affect, it is trying to accelerate the boat (force = mass x acceleration), but is failing to do so because the water resistance is an equivalent force applied in the opposite direction to that of the propeller. If the boat is moving faster (but not accelerating) while all else remains the same, the engine will have to work harder, because the water resistance (force) will be greater. If the boat tries to accelerate above hull speed, or in a constricted channel, at some speed the force required to go faster will be greater than the maximum output the engine the engine is capable of, so the engine will flog its guts out without achieving much more. We've all seen this being done (complete with bank-destroying wash), I'm sure. Going back to the first situation, now add a tow to the slow moving boat. You have thus added the water resistance of the towed boat to the force required to maintain a constant speed. So the engine has to work harder. Now think of punching the tide. To maintain the same speed over the ground, the boat must go faster through the water. So although we have the ground speed of the first case, we have the water resistance of the second, so the engine must work harder. Running the engine in gear when the boat is moored is the limiting case, where you would need huge amounts of engine power to move the boat forward at all, as the resistance is effectively infinite (assuming you don't break the moorings lines, or the bollards, of course). >If ever charging "in gear", to comply with the spirit of the BW >regulations I always spend half the run period in reverse and half in >forward gear just to keep things in balance. That reminds me of a quote from the late Canadian concert pianist, Glenn Gould, which goes approximately: "I often run red lights when driving. But I make up for it by stopping at a few green ones." Running in gear when moored, or indeed putting any other unproductive load on an engine (for any reason, including the belief that it will reduce bore glazing on a generator) is simply gross. It is a 100% waste of fuel, and thus a 100% waste of the money spend to buy that fuel, quite apart from being an environmental offence. Running in gear is seriously likely to annoy the neighbours and damage the waterway, too. (much) More acceptable answers might be to find a useful load (e.g. running the washing machine), or getting a better-designed system for which the generating load is more closely matched to the engine output. Adrian . Adrian Stott 07956-299966
